The present invention relates generally to inkjet printing mechanisms, and more particularly to a positive stop capping system for sealing an inkjet printhead during periods of printing inactivity.
Inkjet printing mechanisms use pens which shoot drops of liquid colorant, referred to generally herein as xe2x80x9cink,xe2x80x9d onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a xe2x80x9cservice stationxe2x80x9d mechanism is mounted within the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. To facilitate priming, some printers have priming caps that are connected to a pumping unit to draw a vacuum on the printhead. During operation, partial occlusions or clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a clearing or purging process known as xe2x80x9cspitting.xe2x80x9d The waste ink is collected at a spitting reservoir portion of the service station, known as a xe2x80x9cspittoon.xe2x80x9d After spitting, uncapping, or occasionally during printing, most service stations have a flexible wiper, or a more rigid spring-loaded wiper, that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.
During periods of printing inactivity, inkjet printheads are typically capped to prevent them from drying out, with the capping reducing evaporation of the ink components, as well as to protect the printhead from contamination due to environmental factors, such as dust, paper particles and the like. To form a good seal, the cap must conform to the printhead and supply enough force against the printhead to limit air transfer. Traditionally, capping has been accomplished using a compliant elastomer that is pressed against the printhead to create a complete seal.
A variety of different tolerance variations occur in the manufacture of inkjet printers, as well as the inkjet cartridges or pens which are installed in these printers, leaving service station designers the task of accommodating these varying tolerances while still providing adequate printhead servicing and high quality print output. The printhead cap designer has a particularly difficult task, needing to maintain an adequate hermetic seal around printheads which may be at varying heights, that is, the distance between the cap sled and the planes within which each of the printhead orifice plates lie. For instance, there may be tolerance variations in the pens themselves, or in the carriage datums, a pen may not be fully seated in the carriage, or a combination of these factors may be work. In multi-pen printers, these variations lead to extra challenges in providing adequate capping force on the higher printheads, while not providing excessive force on the lower printheads. Please note while the terms xe2x80x9chigherxe2x80x9d and xe2x80x9clowerxe2x80x9d are used here by way of reference for a printhead which shoots ink drops downwardly, the same principles apply if the printheads were to shoot the ink drops horizontally or on some other plane, with the term xe2x80x9chighxe2x80x9d being used to indicate a greater cap-to-printhead distance, and the term xe2x80x9clowxe2x80x9d being used to indicate a shorter cap-to-printhead distance. Excessive capping forces may lead to printhead damage, or unseating the pen from the carriage datums. Inadequate capping forces lead to an inadequate cap seal allowing air to enter the capping region and dry out the ink, or lead to severely blocked or occluded nozzles.
Some capping designs used separate springs located under each cap, for instance as shown in U.S. Pat. Nos. 5,867,184 and 5,956,053, currently assigned to the present assignee, the Hewlett-Packard Company. Unfortunately, these systems required separate cap bases, separate elastomeric caps on the bases, cap venting components, and springs for each cap assembly, which not only increases the parts cost, but also the assembly labor cost required to assemble a printer. A unitary capping system was proposed in U.S. Pat. No. 6,220,689 which used a single elastomeric capping structure having four separate caps formed thereon to seal four printheads in a printer. However, this unitary capping design while eliminating the separate caps, cap bases and springs for each printhead, thereby reducing part count, unfortunately was unable to adequately accommodate varying printhead heights to achieve adequate sealing, while avoiding excessive capping forces.